The present invention relates generally to mixing two or more fluids, and more particularly to a static mixing element and to a method for mixing such fluids.
Mixing of two or more fluids is accomplished by conventional dynamic mixers having moving parts and by conventional static mixers having stationary parts. Disadvantages of dynamic mixers include increased maintenance and repair.
Known methods for mixing two or more fluids include placing a conventional static mixer element in a pipe and introducing the fluids into the pipe upstream from the static mixer element. The conventional static mixer element is secured in the pipe or is secured to a section of pipe which then is attached to upstream and downstream pipe sections.
Known static mixer elements include those having perpendicularly interdigitated mixer blades having planar blade surfaces inclined forty-five degrees to the direction of fluid flow. Typically the fluids are liquids. The fluids are mixed as they pass through the openings between the mixer blades. Mixing is improved by increasing the length of the static mixer element or by adding additional static mixer elements, but this increases pressure drop. High-viscosity fluids are typically more difficult to mix than low-viscosity fluids. It is also difficult to mix a low-viscosity fluid and a highly-viscous fluid.
What is needed is an apparatus and an efficient method for better mixing together of two or more fluids, especially when the fluids have widely disparate viscosities such as when one fluid has low viscosity and another fluid is highly-viscous.
A first expression of a preferred embodiment of the invention is for a static mixer element. The static mixer element includes a directional flow axis and interdigitated static mixer blades. The directional flow axis passes through the center of gravity of the static mixer element and points in an intended downstream direction opposite to an intended upstream direction. The static mixer blades each have a concave side facing generally in the intended upstream direction at an acute angle with respect to the intended upstream direction. A plane perpendicular to, and moving in the direction of, the directional flow axis will first and generally simultaneously strike a subset of the static mixer blades. Some of the static mixer blades of the subset are positioned at a positive acute angle with respect to the intended upstream direction. The remainder of the static mixer blades of the subset are positioned at a negative acute angle with respect to the intended upstream direction.
A second expression of a preferred embodiment of the invention is for a static mixer element. The static mixer element includes a directional flow axis, a first group of spaced-apart and generally-aligned blade layers, and a second group of spaced-apart and generally-aligned blade layers. The directional flow axis passes through the center of gravity of the static mixer element and points in an intended downstream direction opposite to an intended upstream direction. Each blade layer of the first group has spaced-apart and generally-aligned static mixer blades each having a concave side facing generally in the intended upstream direction at an acute angle with respect to the intended upstream direction. The blade layers of the second group are aligned generally perpendicular to the blade layers of the first group. Each blade layer of the second group has spaced-apart and generally-aligned static mixer blades each having a concave side facing generally in the intended upstream direction at an acute angle with respect to the intended upstream direction. The static mixer blades of the blade layers of the second group are interdigitated with, and connected to, the static mixer blades of the blade layers of the first group. A plane perpendicular to, and moving in the direction of, the directional flow axis will first and generally simultaneously strike at least one static mixer blade from each of at least two blade layers of each of the first and second groups.
A first expression of a preferred method of the invention is for mixing first and second fluids and includes steps a) through e). Step a) includes obtaining a pipe. Step b) includes obtaining a static mixer element, wherein the static mixer element includes a directional flow axis and a multiplicity of interdigitated static mixer blades. The directional flow axis passes through the center of gravity of the static mixer element and points in an intended downstream direction opposite to an intended upstream direction. The static mixer blades each have a concave side facing generally in the intended upstream direction at an acute angle with respect to the intended upstream direction. Step c) includes positioning the static mixer element in the pipe with the directional flow axis pointing downstream. Step d) includes placing the first fluid in the pipe upstream from the static mixer element. Step e) includes placing the second fluid in the pipe upstream from the static mixer element.
A second expression of a preferred method of the invention is for mixing first and second fluids and includes steps a) through e). Step a) includes obtaining a pipe. Step b) includes obtaining a static mixer element identical to that described in the second previous paragraph. Step c) includes positioning the static mixer element in the pipe with the directional flow axis pointing downstream. Step d) includes placing the first fluid in the pipe upstream from the static mixer element. Step e) includes placing the second fluid in the pipe upstream from the static mixer element.
A second preferred embodiment of the invention is for a static mixer element assembly which includes first and second static mixer elements. The first static mixer element is identical to the static mixer element of the above-described second expression of a preferred embodiment of the invention. The second static mixer element is substantially identical to the first static mixer element, is positioned to have its directional flow axis substantially superimposed on the directional flow axis of the first static mixer element, is rotated generally ninety degrees with respect to the first static mixer element about the directional flow axis of the first static mixer element, and is positioned proximate the first static mixer element.
A second preferred method of the invention is for mixing first and second fluids and includes the steps of the above-described second expression of a preferred method of the invention, wherein xe2x80x9cfirst static mixer elementxe2x80x9d replaces xe2x80x9cstatic mixer elementxe2x80x9d. The second preferred method also includes several additional steps. A first additional step includes obtaining a second static mixer element substantially identical to the first static mixer element. A second additional step includes positioning the second static mixer element in the pipe downstream of the first static mixer element with the directional flow axis of the second static mixer element pointing downstream and with the second static mixer element rotated generally ninety degrees with respect to the first static mixer element about the directional flow axis of the first static mixer element.
Several benefits and advantages are derived from the invention. Using static mixer blades having a concave surface facing generally upstream at an acute angle resulted in better mixing than that achieved using the flat blades of the prior art. In experiments involving mixing water and corn syrup, the curved blades of the invention showed about a twelve percent improvement in mixedness over the flat blades of the prior art for vertical mixing (i.e., when the two fluids flowed vertically upward) and showed over a thirty percent improvement in mixedness for horizontal mixing (i.e., when the two fluids flowed horizontally). Even a twelve percent improvement is significant and means that eleven static mixer elements having the curved blades of the invention would provide the same or better mixedness as twelve static mixer elements having the flat blades of the prior art. The pressure drop from the curved blades of the invention was about five percent lower than that of the flat blades of the prior art. The lower pressure drop can result in an increased throughput in those applications where it is desirable to minimize pressure drop for a given static mixer element length and means a static mixer element having a shorter length in those applications where length, instead of lower pressure drop, is the critical design parameter. It is noted that comparisons of the mixing performance of curved and flat blades were made for static mixer elements of identical length (measured along the directional flow axis) having the same blade intersection area and the same interface area between blades. Applicants discovered that there were more droplets of low-viscosity fluid (the water) forced to the pipe walls by cross-stream flow by the curved blades of the invention than by the flat blades of the prior art. Applicants also discovered that the droplets of low-viscosity fluid (the water) tended to migrate around the flat blades of the prior art nearer to the center portion of the pipe rather than being driven in cross-stream flow toward the pipe wall. This has led Applicants to theorize that improvement in cross-stream flow toward the pipe wall accounts for the improvement in mixedness of the curved blades of the invention.